Artifacts on cube after modulo on coordinates
The Modulo node, when used with vector input repeats the output values at regular intervals, in this case $1$ unit.

You are using Generated texture coordinates as input which uses the objects size for coordinates, meaning it that maximum X,Y or Z are always considered at coordinate $1$ regardless off absolute dimensions.

Since in this case geometry is a cube, there is a planar face at precisely the coordinate X=$1$ which is the boundary point exactly where the Modulo pattern repeats itself and goes from black to white.

And that is what you are seeing, it is not Z-Fighting per-se but it is exactly the same kind of mathematical imprecision computers can't really solve well.